The present invention relates generally to mobile systems that supply alternating-current power, and further relates to ground fault circuit interrupter (GFCI) protection for such systems.
Mobile electric power supply systems are often utilized to provide electric power at various recreation, campground, work, or construction sites where electric power from a public utility may not be readily available. These mobile systems can include portable generators such as are used residentially for backup power, wheeled trailers that can be releasibly hitched to automotive vehicles for higher wattage applications, as well as other generators integrated into a vehicle. See, for example, U.S. Pat. No. 2,733,661 issued Feb. 7, 1956 to Surgi; U.S. Pat. No. 2,898,542 issued Aug. 4, 1959 to Wasko et al.; and U.S. Pat. No. 4,556,247 issued Dec. 3, 1985 to Mahaffy. Once the mobile power supply system is moved to a particular site, one or more electrical loads can then be plugged into or otherwise electrically interfaced with the system and operated as needed. Such electrical loads may include, for example, power tools, commercial equipment, or residential appliances (such as in the event of use of the system as a backup generator).
Ground fault circuit interrupters (GFCIs) are used in utility power applications to protect against leakage currents that flow through ground rather than back through the source""s neutral line. They are commonly found in residential settings where the utility power is used to operate household appliances. GFCI devices commonly include a differential current transformer, control circuit, and a circuit breaker device. The differential current transformer itself can be implemented as a toroidal core with the power line conductors being used as primaries and a secondary winding being connected to the control circuit that is, in turn, connected to the circuit breaker device. Ground fault or leakage currents are detected by the control circuit by sensing the difference in current magnitude between the outgoing current and the return current. When this difference, or imbalance, exceeds a preset threshold, the control circuit trips the circuit breaker device which open circuits the hot conductor.
More specifically, current sensing by the GFCI circuit can be implemented by threading the hot and neutral conductors through the core so that the vector sum of the currents in the hot conductor (or conductors for multi-phase) and the neutral conductor is normally zero. As a result, the magnetic flux generated by the current flowing through the hot conductor (or conductors for multi-phase) and the neutral conductor cancel each other out. Where no ground fault or leakage exists, the current flowing through the hot conductor(s) and the neutral conductor results in a net flux of zero. However, where current is able to follow a return path other than through the design conductor paths, such as through a ground path, the net current through the conductors will not be zero, thereby resulting in a net flux in the core that is sensed by the control circuit via the secondary winding. The control circuit compares the magnitude of this sensed current imbalance and trips the circuit breaker device if that magnitude exceeds the preset trip level. Examples of different GFCI circuits are disclosed in U.S. Pat. No. 3,213,321 issued Oct. 19, 1965 to Dalziel; U.S. Pat. No. 4,150,411 issued Apr. 17, 1979 to Howell; and U.S. Pat. No. 4,180,841 issued Dec. 25, 1979 to Engel.
For mobile electric power supply systems of the type noted above, the use of GFCIs can be problematic because some of the electrical loads powered by the mobile system may have grounded neutrals, while others may not. For example, in residential housing the neutral line is typically connected to earth ground via a metal wire and/or stake, whereas a typical power tool (which may or may not include a ground wire) will normally have an ungrounded neutral. For the power tools use, incorporating GFCI protection into the mobile system is desirable to protect against ground faults. However, if the mobile system is connected to the main circuit breaker box of a house for the purpose of delivering back-up electric power during a local power blackout, the circuit breaker device of the GFCI within the system may trip and cut off the electric power being delivered. This can occur because some electric current may be diverted from returning to the system via the neutral line conductor and is instead returned to the system via the legitimate electrically conductive grounding path. Thus, tripping may occur even in the absence of an undesired or non-legitimate electric current leakage path back to the system. This type of GFCI tripping is often referred to as false tripping or nuisance tripping. Further complicating this scenario is that whenever the GFCI trips in such a configuration, it can be difficult to determine whether the GFCI was (1) falsely tripped due to electric current returning along the legitimate electrically conductive grounding path, (2) legitimately tripped due to electric current returning along an undesired leakage path, or (3) tripped due to a combination of both.
Accordingly, it is a general object of the invention to provide a mobile electric power supply system that is capable of providing electric power with GFCI protection for an electrical load having an ungrounded neutral conductor and that is also capable of providing electric power for an electrical load having a purposely grounded neutral conductor without causing nuisance tripping of the GFCI circuit.
The present invention provides a mobile system for supplying electric power to an electrical load. The mobile system includes an electric power source, first and second receptacles coupled to the power source via two or more power line conductors, a ground fault circuit interrupter (GFCI) for ground fault protection of the second receptacle, and a switch device for use in switching power from the electric power source to either the first (unprotected) receptacle or the second (protected) receptacle. The GFCI is electrically connected to the power line conductors and has a circuit breaker element that is series-connected between the electric power source and the second receptacle. The switch device is electrically coupled between the electric power source and the first and second receptacles, and is connected in series with the GFCI such that current flowing through the circuit breaker element flows through the switch device. The switch device can be switched between a first state, in which the first receptacle receives power from the electric power source that is unprotected by the GFCI, and a second state, in which the second receptacle receives power from the electric power source that is protected by the ground fault circuit interrupter against a current imbalance in the power line conductors. Preferably, the switch device includes a manually activated switch for switching power between the first and second receptacles. With this configuration, a user can supply power to the first receptacle in which case the GFCI is either disabled or bypassed, or can supply power to the second receptacle which will then be ground fault protected by the GFCI. This allows the mobile system to be used for supplying power to loads having a grounded neutral without nuisance tripping of the GFCI while enabling the use of GFCI protection for those loads having an ungrounded neutral.
For the first receptacle used with loads having grounded neutrals, the GFCI can be defeated in various ways. For example, the switch device can be located downstream of the GFCI with the GFCI being disabled when the switch device is set to provide power to the first receptacle. In this way, the current flows through the GFCI, however, its internal control circuit is disabled from tripping the circuit breaker element. Alternatively, the switch device can be placed in circuit before the GFCI, with the GFCI being connected in series between the switch device and second receptacle while the power lines from the switch device to the first receptacle bypass the GFCI altogether.
The mobile system can be implemented as, for example, a portable generator, wheeled trailer, or automotive vehicle. Where a vehicle is used, the electric power source can comprise a vehicle battery and an inverter for generating single or multi-phase AC power. Preferably, the electric power source and GFCI are located in close proximity on the vehicle to minimize the length of non-GFCI protected power line runs. The receptacle can then be located onboard the vehicle remote from the GFCI circuit.
The switch device preferably includes either a manually activated switch to directly switch power between the first and second receptacles, or a lower amperage manually activated switch in combination with a relay that performs the power line switching. When used on a vehicle, the manual switch can be located either inside or outside the vehicle cabin space. Where single phase power is utilized, the switch device can be a single pole device and, where two-phase power is used, a double pole device will preferably be used.
The mobile system preferably includes an indicator circuit having an LED or other visual or audible warning that is activated when power is supplied to the non-GFCI protected first receptacle.